No, it's not, it's Linux but no court has actually said it is. Just compile a binary in linux statically, copy it to ESXi and it'll run. That does not work (out-of-the-box) on FreeBSD.
Use ZFS and install PVE along the VMs, its space requirements are negligible.
If you use a enterprise grade, write intensive/industrial grade USB stick, it will work flawlessly.